Electrical crystal unit for use at microwave frequencies



Oct. 3l, 1967 T. S. SAAD ELECTRICAL CRYSTAL UNIT FOR USE AT MICROWAVE FREQUENCIES Original Filed Jan. 28, 1955 ATTORNEYv United safes Patent one@ 3,350,655 Patented Oct. 31, 1967 3,350,655 ELECTRICAL CRYSTAL UNIT FOR USE AT MICROWAVE FREQUENCIES Theodore S. Saad, Westwood, Mass., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Continuation of abandoned appiicatien Ser. No. 167,02, Jan. 18, 1962, which is a division of application Ser. No. 484,763, Jan. 23, 1955, now Patent No. 3,028,560, dated Apr. 3, 1962. This application Nov. 16, 1965, Ser. No. 511,578 The portion of the term of the patent subsequent to Apr. 3, 1979, has been disclaimed 3 Claims. (Cl. 329-162) VThis application is a continuation of earlier filed application Ser. No. 167,092, filed Jan. 18, 1962, which application is a division of earlier filed application Ser. No. 484,763, filed Jan. 28, 1955, now Patent No. 3,028,560 issued Apr. 3, 1962.

The present invention relates to microwave apparatus, and more particularly to an improved coaxial crystal cartridge which may be employed as a rectifier, mixer, and the like at microwave frequencies.

For the mixer of superheterodyne receivers used at microwave frequencies, it is common practice to employ a crystal diode, rather than a vac-uum tube type mixer. Crystals, commonly although not necessarily silicon, afford more favorable signal to noise ratios than the triode or pentode `mixers used at lower frequencies. Furthermore, crystals by reason of their small physical size and simple configuration are better suited to effective incorporation Within waveguide or coaxial cable transmission systems,

In such systems wherein a crystal mixer is employed, it is necesary to provide suitable connections for the incoming signal (hereinafter termed the R.F. input), the oscillator signal (LO. or local oscillator), and for the output signal (LF. signal) resulting from the beating or mixing of the oscillator frequency with the signal frequency. In the conventional coaxial crystal cartridge, the crystal is secured to one face of a metal plug, termed the back plug, that is, in direct metallic contact with the shell. The catwhisker that contacts the crystal is carried by a pin that extends through and is supported by an insulating bead fitting within the shell.

In such conventional cartridges, it is necessary to ernploy a mounting for the cartridge which enables the pin to serve as the input connection for the R.F. signal to the crystal, and also for the LF. or video output from the crystal. This necessitates the use of an R.F. choke on the output arm of the mounting, to keep the R.F. signal out of the input to the LF; or video amplifier. Since the choke is effective only over a relatively restricted frequency range, the use of an R.F. choke limits the frequency range over which the assembly is operative, so that broad band operation is not satisfactory.

In addition to the problems involved in the RF. choke,

the conventional crystal cartridge necessitates special provisions in the mounting for the separate LF. output from the pin carrying the Whisker. The conventional crystal mounting also presents connection problems when the crystal is to be used as a simple detector. y' It is accordingly an object of the invention to provide a new and improved coaxial crystal cartridge for crystals operating at microwave frequencies, wherein the connections may be made simply and effectively, both for mixer and for detection applications.

It is another object of the invention to provide a coaxial crystal cartridge which makes possible the use of mountings of rsimplified construction and affording wide band operation at microwave frequencies.

, Still another object of the invention is to provide a coaxial crystal cartridge having means Within the cartridge for effectively isolating the RF. input from the I.F. or video output, so as to eleminate the necessity for isolating means in the [mounting structure for the cartridge.

A further object of the invention is to provide a coaxial crystal cartridge that may be effectively employed with Wave guide, with coaxiallines, and with microstrp feed systems to afford simple and compact installation and efficient operation at microwave frequencies.

YThe invention has as still another object the provision of a crystal cartridge wherein a self-contained direct current return path may readily be provided when desired.

With these and other vobjects in view, the present invention contemplates as a feature thereof a crystal cartridge wherein the plug is electrically isolated from the shell in such a manner as to provide determined reactive characteristics.

More specifically, it is a feature of the invention to provide, between the plug and the shell of the cartridge, a capacitive reactance such that there exists an effective by-pass to RF. frequencies While affording a substantially open circuit for signals Within the I.F. or video range. By providing for connection to the LF. system at the outer end of the plug, the crystal may effectively be coupled to the I.F. amplifier While the R.F. signal is substantially bypassed. The elimination of the R.F. choke not only simplifies the construction of the mounting for the cartridge, .but also permits relatively broad band operation. Since the new crystal cartridge provides its own output connection at the plug end as an integral part of the cartridge, independent coaxial input and output connections thereby are provided at opposite ends of the device. Thus the device may conveniently be termed a feed-through crystal cartridge and such designation is hereinafter employed.

In the drawings illustrating the invention according to its several embodiments,

FIG. 1 is a View in sectional elevation on a somewhat enlarged scale, showing the novel feed-through cartridge construction.

FIG. 2 illustrates the use of the as a mixer in a coaxial line.

FIG. 3 shows the cartridge mounted in a coaxial line for use as a simple detector.

FIG. 4 illustrates the manner of mounting the feedthrough cartridge for use as a waveguide mixer.

FIG. 5 shows in sectional elevation a slightly modified form of lfeed-through crystal cartridge, embodying an integral D.C. return, with the cartridge shown mounted for use with so-called microstrip or microwave strip feed-through crystal transmission line.

FIG. 6 illustrates in sectional elevation a feed-through crystal device alternative to that of FIG. 1.

FIG. 7 illustrates in sectional elevation another feedthrough crystal device.

The crystal cartridge illustrated in FIG. 1 comprises a shell 12 of conductive material with an insulating bead 14 through which extends the pin 16. The diameter of the shell is such as to permit appropriate connection to the outer conductor of coaxial cable to which the device may be connected. The outer end of the pin is reduced at 18 to make telescoping connection with the center conductor of the cable. The other end of the pin carries the cat- Whisker 20 which may be of the usual resilient wire construction, pointed for contact With the crystal.

The crystal element 22, which may be of silicon, germanium or other suitable semi-conductor exhibiting appropriate rectifying properties, is mounted, as by soft soldering, to the inner face of the metallic back Iplug 24. The spacing between back plug 24 and bead 14 is such as to provide the desired contact pressure between Whisker and semi-conductor surface when the point of the Whisker 3 has been located on the desired spot on the semi-conductor surface.

The back plug 24 of the crystal unit unlike conventional coaxial cartridge constructions, is not in direct metallic contact with the shell 12 (which results in zero impedance at all frequencies including D.C.), 4but instead is insulated therefrom in a particular manner and in such fashion as to afford novel and useful results. In the coaxial crystal device of FIG. 1, the back plug is insulated from the sleeve by a thin film or sleeve 30 of dielectric material of effective insulating properties. By way of example, the insulating sleeve may be an epoxy resin, a polyester resin, a copolymer comprising polymerized dichlorostyrene such as identified by the trade mark Styron, or a tetraiiuoroethylene resin such as that designated by the trade mark Teflon. For convenience, the dielectric material may be disposed between the plug 24 and a thin metallic Jacket or sleeve 32 to facilitate insertion of the insulated plug within the main shell 12 which forms the envelope of the cartridge.

The insulation of the plug from the shell results in D.C. isolation of the crystal 4and its supporting plug from the shell or outer conductor 12. More importantly, the effect of the insulating sleeve is to constitute a condenser of small capacity which may be made effective to by-pass or short-circuit certain frequencies while having no appreciable effect n relatively lower frequencies. Thus, where the crystal is employed as a mixer or a video detector, the insulating sleeve intermediate the shell and plug results in a low-capacity condenser of coaxial configuration, the capacity of which may be -made such as to by-pass effectively the R.F. signal while causin-g no substantial attenuation of the desired LF. or video output slgnal resulting from mixer action.

Because of this selective by-pass effect, it becomes possible to derive the crystal output directly from the back plug 24, and for this purpose the plug is provided with a terminal 36, to which the center conductor of coaxial cable may be connected for feeding and I F. amplifier or the like. The crystal device thus becomes a true coaxial element with feed-through from one end to the other, the input being applied to the pin end and the rectified or mixer output, as the case may be, derived coaxially from the other end and with an integral coaxial RF. by-pass condenser directly associated with the crystal support.

By way of example of the capacity values readily attainable with the integral coaxial by-pass condenser, a device employing a shell of 0.188 inside diameter and an insulating sleeve 0.004 thick, formed of an epoxy resin havin-g a dielectric constant of 2.0, the plug length being approximately 0.188 long, provides a capacitance of approximately 12 auf. This is a value that provides a reactance of the order of an ohm or less, at frequencies higher than about lOOOM c.p.s. so as to by-pass effectively the R.F. energy. The I.F. output, being at considerably lower frequency, is not seriously attenuated, as the small capacitance presents a relatively high impedance at such frequencies.

The utilization of the feed-through crystal device in mixer applications is illustrated in FIG. 2. In this embodiment, which is for use with coaxial lines, the feed-through crystal cartridge, indicated at 50, is mounted in arm 52 of the coaxial mixer. Arm 54 connects to the coaxial line that feeds the R.F. signal to the crystal, while arm 56 injects the local oscillator signal, the oscillator being connected at 58 with the injection controlled in the usual manner by adjustment knob 60. The center conductor 62 connects to the pin of the cartridge in the manner described in FIG. 1, while the LF. output from the mixer is obtained from the pin 64 on the crystal back plug.

The feed-through cartridgeis likewise effective in crystal rectiiiers and detectors at microwave frequencies. FIG. 3 illustrates the improved crystal device mounted at the end of a coaxial cable to serve as a simple detector. The R.F. signal is supplied by coaxial line having outer conductor 70 and center conductor 72. A tapered fitting 7'4 provides a match to the crystal 76, the body of which is received in the end of the fitting.

Instead of requiring the conventional video output network which unduly complicates the crystal connection to the end of the line, the new feed-through crystal cartridge construction makes it possible to derive the video output directly from the back plug terminal 78, with the RF. energy effectively by-passed by the integral coaxial condenser formed by the insulating dielectric sleeve between body and back plug, as shown in FIG. 1. For the D-C return, the usual connection may be provided in the coaxial line, or the integral D-C return hereinafter described and illustrated may conveniently be employed.

The advantages of the feed-through crystal when utilized as a mixer in waveguide installations will be readily apparent from FIG. 4. Here the mixer waveguide section is indicated at 80, with flange 82 for connection to the waveguide through which the R.F. signal land the local oscillator energy are supplied. The crystal unit is shown at 84 mounted in coaxial fitting 86 on one wall of the guide, with the center connection extending to the opposite wall of the waveguide by conductor to provide the D-C return.

Here again, instead of requiring an elaborate choke construction in the waveguide mounting land connected via the center conductor 90 to provide the I.F. output, it is merely necessary to connect to the terminal -38 on the specially-mounted back plug, as shown in FIG. l and described. Like the coaxial mixer construction of FIG. 2, the RF. energy is effectively by-passed, for a wide range of frequencies, by the coaxial by-pass condenser that is an integral part of the crystal cartridge.

A further example of the use of the feed-through crystal as embodied in microstrip waveguide is shown in FIG. 5. I-Iere the waveguide elements are the strips 91 and 92, with insulated spacing material 94. The coaxial fitting 96 secured to strip 91 provides a ymounting for the shell 98 of the cartridge, with the center pin 100 connected to the bottom 4strip 92 through short conductor 102.

In this embodiment, the back plug 104 on which the crystal 106 is mounted is spaced from the shell 98 solely by the dielectric sleeve 108. For dielectric materials having physical properties that permit convenient assembly of the plug within the shell, the separate metal sleeve 32 illustrated in FIG. 1 may be omitted. As in the other embodiments, the back plug carries the coaxial terminal 110 that provides the feedthrough output connection.

This embodiment likewise serves to illustrate a further feature of the invention, it being understood that such feature is in no way -limited to cartridges for microstrip applications but may be used wherever crystals having an integral D-C return are desired, for example, with probetype connections where no direct return exists. In this construction, the Whisker 112 is provided with an extension 114, or, alternatively, a separate lead is provided, from the inner end of Whisker pin 100. This lead extends radially to the shell, and then along the inner wall thereof, being firmly held in position between the insulating bead 116 and the shell 98. This D-C return is readily provided at the time of assembling the crystal device, the lead being laid along the outside of the insulating bead and the bead then forced into place within the shell.

In the event, in this microstrip application of the feedthrou-gh crystal, no integral D-C return is provided, an external return may be employed as indicated at 120 by the dotted line short across the strips 91, 92 at the end of the arm.

While the invention has so far been described as embodied in constructions in which the crystal element is carried by the plug, and the Whisker by the insulating bead, this is of course not the only arrangement possible.

FIG. 6 illustrates the feed-through crystal features in a crystal cartridge in which the crystal element and cat- Whisker are interchanged from their positions shown in FIG. 1.

In this embodiment the -shell is indicated at 121, the conductive plug element or back plug at 122, and the insulating bead at 124. The crystal 126 is soft soldered to `a pin 130 extending through the bead to provide the coaxial connection to the crystal element thereon. The catwhisker 132 is carried by the conductive plug 22, with terminal 134 on the outer end of the plug providing the feed-through coaxial connection as previously described.

The conductive plug, as in the other embodiments, is electrically insulated from the shell (with respect to D.C.) by a thin film or sleeve of dielectric material 136. The sleeve thickness is such as to retain the plug securely in position, with the gap between the inner surface of the metallic shell and the outer surface of the plug providing the required capacitive reactance for proper by-passing at the operatin-g frequency. No integral D C. return is shown in this embodiment, but one may readily be provided, when required, by extending a conductor from the crystal supporting pin 130 into jammin-g relation between bead and shell, as is done with the D.C. return 1,14 in FIG. 5 from the whisker support.

FIG. 7 illustrates a device embodying a combination of various features shown in FIGS. 1, 5, and 6. Within the conductive shell 140 of the device are mounted the insulating bead 141 and the conductive back plug 142. The crystal element 143 is mounted on the conductive supporting pin 144 extending through the bead, and the catwhisker 145 is carried by the conductive plug. The conductive plug is electrically insulated from the shell (with respect to D.C.) by a thin sleeve of dielectric material 146. A thin metallic sleeve 147 in contact with the -shell surrounds the `dielectric material. An integral D.C. return is provided by a conductor 148 extending from the crystal supporting pin |144 into jamming relation between the bead 141 and shell 140.

FIGS. 2, 3, 4 and 5 of `the drawings are to be considered as merely illustrative of typical mounting arrangement-s wherein the ladvantages of the novel feed-through crystal :are effectively realized, when compared with the relatively complex arrangements heretofore employed. Other forms and construtcions for utilizing the integral by-pass crystal cartridge or the present invention will be apparent to those skilled in the art.

Tests of apparatus embodying crystal cartridges constructed in accordance with the invention have indicated that wide band performance is effectively realized. Thus, measurements of tangential sensitivity of video detector units have shown relatively high and uniform response characteristics over a frequency range from 1000 to over 10,000M c.p.s. Thus it appears that effective R.F. bypassing occurs over a very substantial frequency range, =as compared with the uneven or limited response that results when conventional crystal devices and mounting means therefor involvin-g the usual R.F. choke or other narrow-band means are employed.

I claim:

1. A crystal cartridge for microwave apparatus cornprising a conductive shell, a crystal element and `a cooperating catwhisker element within said shell, a conductive back plug on which the catwhisker element is mounted, an insulating support for said crystal element within and directly engaging said shell and supported thereby, a coaxial input extending through and mounted to the insulating support and making connection to said crystal element, a coaxial output connection on the back plug, said plug and said insulating support having a fixed spacing therebetween which establishes rectifying contact between said crystal and catwhisker elements, said fixed spacing being maintained by the secure mounting of said plug and said insulating support within said conductive shell, a thin metal sleeve surrounding the back plug and in electrical contact with the interior of the shell, and dielectric material between the metal sleeve and back plug to form a coaxial capacitor having low impedance at Imicrowave frequencies.

2. A crystal cartridge for microwave frequencies comprising a conductive shell, a crystal element and a cooperating catwhisker element within the shell, a conductive plug at a position within the shell adjacent one end thereof on which said catwhisker element is lmounted, means securing the plug in said position, said means comprising a thin insulating sleeve of dielectric material intermediate the plug and shell, the plug, the shell and the means securing the plug in position forming -a coaxial R.F. by-pass condenser, a conductive pin on which the 4crystal element is mounted, an insulating support for said pin directly engaging the shell and supported thereby at a position therein spaced along the lshell at a fixed distance from the plug and said insulating sleeve, said pin extending through and secured in the insulating support, the crystal element being mounted at the end of said pin lying within the space between the support and the plug, the fixed distance between the insulating support and the conductive plug establishing rectifying contact between the crystal and catwhisker elements, and a conductor extending from the crystal element end of the pin into contact with the shell providing a D.C. path from crystal element to shell, the shell and plug affording coaxial output connections and the shell and pin affording coaxial input connections for the cartridge.

3. A crystal cartridge for microwave frequencies comprising a conductive shell, a crystal element and a cooperating catwhisker element within the shell, a conductive plug at a position within the shell adjacent one end thereof on which said catwhisker element is mounted, means securing the plug in said position, said means comprising a thin metal sleeve surrounding the plug and in electrical contact with the interior of the shell and a thin insulating sleeve of dielectric material between the metal sleeve and the plug, the plug, the shell and the means securing the plug in position forming a coaxial R.F. bypass condenser, a conductive pin on which Vthe crystal element is mounted, an insulating support for said pin directly engaging the shell and supported thereby at a position therein spaced along the shell ata fixed distance from the plug and said insulating sleeve, said pin extending `through and secured in the insulating support, the crystal element being mounted at the end of said pin lying within the space between the support and the plug, the fixed distance between the insulating support and the conductive plug establishing rectifying cont-act between the crystal and catwhisker elements, and a conductor extending from the crystal element end of the pin into contact with the shell providing a D.C. path from crystal element to shell, the shell and plug affording coaxial output connections and the shell and pin affording coaxial input connections from the cartridge.

References Cited UNITED STATES PATENTS 3,028,560 4/1962 Saad 329-162 ALFRED L. BRODY, Primary Examiner. ROY LAKE, Examiner. 

1. A CRYSTAL CARTRIDGE FOR MICROWAVE APPARATUS COMPRISING A CONDUCTIVE SHELL, A CRYSTAL ELEMENT AND A COOPERATING CATWHISKER ELEMENT WITHIN SAID SHELL, A CONDUCTIVE BACK PLUG ON WHICH THE CATWHISKER ELEMENT IS MOUNTED, AN INSULATING SUPPORT FOR SAID CRYSTAL ELEMENT WITHIN AND DIRECTLY ENGAGING SAID SHELL AND SUPPORTED THEREBY, A COAXIAL INPUT EXTENDING THROUGH AND MOUNTED TO THE INSULATING SUPPORT AND MAKING CONNECTION TO SAID CRYSTAL ELEMENT, A COAXIAL OUTPUT CONNECTION ON THE BACK PLUG, SAID PLUG AND SAID INSULATING SUPPORT HAVING A FIXED SPACING THEREBETWEEN WHICH ESTABLISHES ECTIFYING CONTACT BETWEEN SAID CRYSTAL AND CATWHISKER ELEMENTS, SAID FIXED SPACING BEING MAINTAINED BY THE SECURE MOUNTING OF SAID PLUG AND SAID INSULATING SUPPORT WITHIN SAID CONDUCTIVE SHELL, A THIN METAL SLEEVE SURROUNDING THE BACK PLUG AND IN ELECTRICAL CONTACT WITH THE INTERIOR OF THE SHELL, AND DIELECTRIC MATERIAL BETWEEN THE METAL SLEEVE AND BACK PLUG TO FORM A COAXIAL CAPACITOR HAVING LOW IMPEDANCE AT MICROWAVE FREQUENCIES. 